The objective of this research is to analyze normal and abnormal mechanisms of development through use of mutant genes of the mouse. The mutations to be studied include: (1) tight skin (Tsk), a dominant mutation causing extensive hyperplasia of the loose connective tissue, enlarged heart and lungs, and abnormally long cartilages and long bones; (2) motheaten (me) a recessive mutation causing extensive skin lesions, severe pneumonitis, and early death, and showing evidence of an immune deficiency. The methods will include light and electron microscopy and the experimental methods appropriate for immunological studies.